1. Field of the Invention
The present invention relates to a zoom lens system of a high zoom ratio greater than 2.3 ranging from a telephoto to wide field angle for use in a still camera, such as 35 mm SLR cameras.
2. Description of the Prior Art
In this field of art, there has been various attempts to provide a zoom lens system of a relatively high zoom ratio ranging to wide field angle. Prior art zoom lens systems attempting to achieve these results can be roughly classified into two types, the first one having a negative front lens group, while the second one having a positive front lens group.
The first type is in its nature favorable to the purpose of providing the wide angle zoom lens system, while it is unfavorable to the reducing the total length thereby providing a compactness to the lens system. A most popular and simplified zoom lens system that can be classified as a first type is a so-called two-group zoom lens system consisting of a negative front lens group and a positive rear lens group. Although such a two-group zoom lens system is suitable for a limited zoom ratio, such as 2, it is difficult to further increase the zoom ratio. In other words, if the zoom ratio in the two-group zoom lens system is attempted to be increased so as to range to a focal length greater than 85 mm in terms of a 35 mm SLR camera, the manufacturing errors of air spaces between the lens elements greatly influence the correction of spherical aberration in the longer focal length range to thereby fail in providing any practical and reliable product.
On the other hand, in the second type of zoom lens system with the front positive lens group, if the zoom range is extended to reach a focal length less than the length of a diagonal line of the effective image plane, the focusing by means of moving the front positive lens group is difficult or the diameter of the front positive lens group is excessively increased. A typical example of a zoom lens system classified into the second type is a so-called four-group zoom lens system consisting of a front focusing lens group, a variator lens group, a compensator lens group and a relay lens group. In such a four-group zoom lens system, if the zoom range is attempted to be extended to a focal length less than 40 mm with the focusing capability by means of the front lens group and the compactness of the whole lens system maintained, the focal lengths of the front lens group and the variator lens group become excessively short to cause various abberrations which are difficult to be sufficiently corrected.
Examples of patented prior art literature can be found in U.S. Pat. No. 4,256,381 and U.S. Pat. No. 4,299,454.
The prior art is still seeking to provide a relatively compact and economical zoom lens system having a relatively wide range of zooming into the wide angle range with adequate optical correction.